Reference
一、总结
1.jdk 1.8.0
2.引用级别
- FinalReference，不对外提供使用，类的访问权限为默认 protect，即使抛出 out of memory 异常也不会回收所占内存
- SoftReference，在内存不够用时，在抛出 out of memory 前回收内存空间
- WeakReference，第一次GC不回收，当第二次GC时回收，有限期到第二次GC前都在内存中
- PhantomReference
3.引用对比
| - | 强引用 | 软引用 | 弱引用 | 虚引用 |
| 定义 | 强引用指的是，程序中有直接可达的引用，而不需要通过任何引用对象，如Object obj = new Object();中，obj为强引用 | 软引用，非强引用，但是可以通过软引用对象来访问。软引用的对象，只有在内存不足的时候（抛出OOM异常前），垃圾收集器会决定回收该软引用所指向的对象。软引用通常用于实现内存敏感的缓存。 | 弱引用，非强引用和软引用，但是可以通过弱引用对象来访问。弱引用的对象，不管内存是否足够，只要被垃圾收集器发现，该引用的对象就会被回收。实际的应用见WeakHashMap | 虚引用，该引用必须和引用队列(ReferenceQueue)一起使用，一般用于实现追踪垃圾收集器的回收动作，比如在对象被回收的时候，会调用该对象的finalize方法，在使用虚引用可以实现该动作，也更加安全 |
4.其他
- 子类不可直接继承；处于安全考虑，Reference 类与GC交互；实现方式：构造方法的访问权限为默认，即包外的类不可见
- 对象封装了其它对象的引用，可以和普通的对象一样操作，在一定的限制条件下，支持和垃圾收集器的交互。即可以使用Reference对象来引用其它对象，但是最后还是会被垃圾收集器回收。程序有时候也需要在对象回收后被通知，以告知对象的可达性发生变更
5.流程
二、源码分析
/**
* Abstract base class for reference objects. This class defines the
* operations common to all reference objects. Because reference objects are
* implemented in close cooperation with the garbage collector, this class may
* not be subclassed directly.
*
* @author Mark Reinhold
* @since 1.2
*/
public abstract class Reference<T> {
类的注释：
- Reference 类是引用对象的抽象基类
- Reference 类中定义了引用对象的常用操作
- 由于引用对象是通过与垃圾回收器密切合作来实现的，因此，不能直接为此类创建子类
Reference 类不能被包外的类继承的实现：
- Reference 类的构造方法的访问权限为默认，即同包内的类可见
- 子类的构造方法默认通过 super() 调用父类的构造方法，访问不到父类的构造方法
// 构造方法，两个
// 区别：是否带有 ReferenceQueue 参数
// 不带有 ReferenceQueue 参数的构造方法，当 Reference 被 GC 回收后直接由 Active 状态变为 InActive 状态
Reference(T referent) {
this(referent, null);
}
Reference(T referent, ReferenceQueue<? super T> queue) {
this.referent = referent;
this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
}
// 带有 ReferenceQueue 参数的构造方法，当 Reference 被 GC 回收后会被加入到 GC 自动加入到 pending-Reference list 中，即类中的属性 pengding
private static Reference<Object> pending = null;
// 对应构造方法传入的两个参数
private T referent; /* Treated specially by GC */
volatile ReferenceQueue<? super T> queue;
// 不同状态队列的next元素
/* When active: NULL
* pending: this
* Enqueued: next reference in queue (or this if last)
* Inactive: this
*/
@SuppressWarnings("rawtypes")
Reference next;
- 使用：访问权限为默认，当前类中未使用到该属性；为同包中其他类中调用使用，如：ReferenceQueue 中使用到了
- 作用：Reference 数据结构为链表，用于连接后续的对象
- Reference next ， 在 ReferenceQueue 中指代构造方法传入的参数中的 ReferenceQueue
| Reference状态 | ReferenceQueue中的next的指向 |
| Active | NULL |
| Pending | 本身 |
| Enqueued | 下一个元素或本身（尾部的元素指向本身） |
| Inactive | NULL |
/* A Reference instance is in one of four possible internal states:
*
* Active: Subject to special treatment by the garbage collector. Some
* time after the collector detects that the reachability of the
* referent has changed to the appropriate state, it changes the
* instance's state to either Pending or Inactive, depending upon
* whether or not the instance was registered with a queue when it was
* created. In the former case it also adds the instance to the
* pending-Reference list. Newly-created instances are Active.
*
* Pending: An element of the pending-Reference list, waiting to be
* enqueued by the Reference-handler thread. Unregistered instances
* are never in this state.
*
* Enqueued: An element of the queue with which the instance was
* registered when it was created. When an instance is removed from
* its ReferenceQueue, it is made Inactive. Unregistered instances are
* never in this state.
*
* Inactive: Nothing more to do. Once an instance becomes Inactive its
* state will never change again.
*
* The state is encoded in the queue and next fields as follows:
*
* Active: queue = ReferenceQueue with which instance is registered, or
* ReferenceQueue.NULL if it was not registered with a queue; next =
* null.
*
* Pending: queue = ReferenceQueue with which instance is registered;
* next = this
*
* Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
* in queue, or this if at end of list.
*
* Inactive: queue = ReferenceQueue.NULL; next = this.
*
* With this scheme the collector need only examine the next field in order
* to determine whether a Reference instance requires special treatment: If
* the next field is null then the instance is active; if it is non-null,
* then the collector should treat the instance normally.
*
* To ensure that a concurrent collector can discover active Reference
* objects without interfering with application threads that may apply
* the enqueue() method to those objects, collectors should link
* discovered objects through the discovered field. The discovered
* field is also used for linking Reference objects in the pending list.
*/
// 用于保存对象的引用，GC会根据不同Reference来特别对待，构造方法的参数
private T referent; /* Treated specially by GC */
// 简述：如果需要通知机制，则保存的对对应的队列
// 使用：构造方法的参数
// 作用：创建Reference时，将Queue注册到Reference中，当该Reference所引用的对象
// 被垃圾收集器回收时，会将该Reference放到该队列中，相当于一种通知机制
volatile ReferenceQueue<? super T> queue;
/* When active: next element in a discovered reference list maintained by GC (or this if last)
* pending: next element in the pending list (or null if last)
* otherwise: NULL
*/
// 指向队列中的下一个对象；不同于next，为当前类中使用
transient private Reference<T> discovered; /* used by VM */
/* Object used to synchronize with the garbage collector. The collector
* must acquire this lock at the beginning of each collection cycle. It is
* therefore critical that any code holding this lock complete as quickly
* as possible, allocate no new objects, and avoid calling user code.
*/
static private class Lock { }
private static Lock lock = new Lock();
/* List of References waiting to be enqueued. The collector adds
* References to this list, while the Reference-handler thread removes
* them. This list is protected by the above lock object. The
* list uses the discovered field to link its elements.
*/
// 等待进行 enqueued 操作的对象集合；
// 当 Reference-handler 线程删除元素后，GC 将删除的元素加入此队列中；
// 此集合通过上述的 lock 锁实现线程安全
// 此集合通过 discovered 属性链接本身的元素
// pending队列中的元素由GC自动加入（对象回收后放入此队列中）
private static Reference<Object> pending = null;
由类中的属性得知：内部的数据结果是单链表
ReferenceQueue<? super T> queue 的作用
- queue 通过构造方法传入，若无默认为 null，用于存入注册到队列上的引用对象
- queue 区分不同状态的 Reference ，不同的状态对应不同的queue
Reference有4种状态
- Active：Active状态的Reference会受到GC的特别关注，当GC察觉到引用的可达性变化为其它的状态之后，它的状态将变化为Pending或Inactive，到底转化为Pending状态还是Inactive状态取决于此Reference对象创建时是否注册了queue.如果注册了queue，则将添加此实例到pending-Reference list中。 新创建的Reference实例的状态是Active。
- Pending：在pending-Reference list中等待着被Reference-handler 线程入队列queue中的元素就处于这个状态。没有注册queue的实例是永远不可能到达这一状态
- Enqueued：队列中的对象的状态，当实例被移动到ReferenceQueue外时，Reference的状态为Inactive。没有注册ReferenceQueue的不可能到达这一状态的
- Inactive：一旦一个实例变为Inactive，则这个状态永远都不会再被改变
Reference四种状态对应的队列
- Active queue：刚创建的队列或队列中没有对象注入；next = null ；由构造方法可知，新创建的实例都处于此状态
- Pending queue：
- Enqueued queue：
- Inactive queue：
/* High-priority thread to enqueue pending References
*/
private static class ReferenceHandler extends Thread {
private static void ensureClassInitialized(Class<?> clazz) {
try {
Class.forName(clazz.getName(), true, clazz.getClassLoader());
} catch (ClassNotFoundException e) {
throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
}
}
static {
// pre-load and initialize InterruptedException and Cleaner classes
// so that we don't get into trouble later in the run loop if there's
// memory shortage while loading/initializing them lazily.
ensureClassInitialized(InterruptedException.class);
ensureClassInitialized(Cleaner.class);
}
ReferenceHandler(ThreadGroup g, String name) {
super(g, name);
}
public void run() {
while (true) {
tryHandlePending(true);
}
}
}
/**
* Try handle pending {@link Reference} if there is one.<p>
* Return {@code true} as a hint that there might be another
* {@link Reference} pending or {@code false} when there are no more pending
* {@link Reference}s at the moment and the program can do some other
* useful work instead of looping.
*
* @param waitForNotify if {@code true} and there was no pending
* {@link Reference}, wait until notified from VM
* or interrupted; if {@code false}, return immediately
* when there is no pending {@link Reference}.
* @return {@code true} if there was a {@link Reference} pending and it
* was processed, or we waited for notification and either got it
* or thread was interrupted before being notified;
* {@code false} otherwise.
*/
static boolean tryHandlePending(boolean waitForNotify) {
Reference<Object> r;
Cleaner c; // PhantomReference 的子类
try {
synchronized (lock) {
// pengding Reference Queue 不为空
if (pending != null) {
r = pending; // 将 pending 赋值给 r
// 'instanceof' might throw OutOfMemoryError sometimes
// so do this before un-linking 'r' from the 'pending' chain...
// r 是否是 Cleaner 的实例对象
c = r instanceof Cleaner ? (Cleaner) r : null;
// unlink 'r' from 'pending' chain
// pengding 指向下一个元素
pending = r.discovered;
r.discovered = null;
} else {
// pending Reference Queue 为空
// The waiting on the lock may cause an OutOfMemoryError
// because it may try to allocate exception objects.
if (waitForNotify) {
lock.wait();
}
// retry if waited
return waitForNotify;
}
}
} catch (OutOfMemoryError x) {
// Give other threads CPU time so they hopefully drop some live references
// and GC reclaims some space.
// Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
// persistently throws OOME for some time...
Thread.yield();
// retry
return true;
} catch (InterruptedException x) {
// retry
return true;
}
// Fast path for cleaners
if (c != null) {
c.clean();
return true;
}
ReferenceQueue<? super Object> q = r.queue;
if (q != ReferenceQueue.NULL) q.enqueue(r); // 入队列
return true;
}
static {
ThreadGroup tg = Thread.currentThread().getThreadGroup();
for (ThreadGroup tgn = tg;
tgn != null;
tg = tgn, tgn = tg.getParent());
Thread handler = new ReferenceHandler(tg, "Reference Handler");
/* If there were a special system-only priority greater than
* MAX_PRIORITY, it would be used here
*/
handler.setPriority(Thread.MAX_PRIORITY);
handler.setDaemon(true);
handler.start();
// provide access in SharedSecrets
SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
@Override
public boolean tryHandlePendingReference() {
return tryHandlePending(false);
}
});
}
从源码中可以看出，这个线程在Reference类的static构造块中启动，并且被设置为最高优先级和daemon状态。此线程要做的事情就是不断的的检查pending是否为null，如果pending不为null，则将pending进行enqueue，否则线程进行wait状态。
/**
* Returns this reference object's referent. If this reference object has
* been cleared, either by the program or by the garbage collector, then
* this method returns <code>null</code>.
*
* @return The object to which this reference refers, or
* <code>null</code> if this reference object has been cleared
*/
// 返回当前引用对象的指向的对象；如果对象已经被清除或被GC回收，返回NULL
public T get() {
return this.referent;
}
/**
* Clears this reference object. Invoking this method will not cause this
* object to be enqueued.
*
* <p> This method is invoked only by Java code; when the garbage collector
* clears references it does so directly, without invoking this method.
*/
// 触发这个方法则不会进行入队的操作
public void clear() {
this.referent = null;
}
/* -- Queue operations -- */
/**
* Tells whether or not this reference object has been enqueued, either by
* the program or by the garbage collector. If this reference object was
* not registered with a queue when it was created, then this method will
* always return <code>false</code>.
*
* @return <code>true</code> if and only if this reference object has
* been enqueued
*/
// 判断对象是否已经入队
public boolean isEnqueued() {
return (this.queue == ReferenceQueue.ENQUEUED);
}
/**
* Adds this reference object to the queue with which it is registered,
* if any.
*
* <p> This method is invoked only by Java code; when the garbage collector
* enqueues references it does so directly, without invoking this method.
*
* @return <code>true</code> if this reference object was successfully
* enqueued; <code>false</code> if it was already enqueued or if
* it was not registered with a queue when it was created
*/
// 入队操作
public boolean enqueue() {
return this.queue.enqueue(this);
}
博文参考：
《Java源码分析》：ReferenceQueue、Reference及其子类
Java Reference 源码分析